1. Field of the Invention
The present invention relates to an improved method recovering viscous petroleum from subterranean, viscous petroleum-containing formations including tar sand or bitumen sand deposits. More particularly, the invention relates to the recovery of viscous petroleum from subterranean formations containing same by a preliminary treatment with unheated air followed in situ combustion or low temperature, controlled oxidation oil recovery.
2. Description of the Prior Art
There are many subterranean, petroleum-containing formations in various parts of the world from which substantial amounts of petroleum cannot be recovered because the viscosity of the petroleum is so great that it is essentially immobile at reservoir conditions. Thus even if the formation contains adequate permeability, and an extraneous fluid such as water is introduced into the formation to drive the petroleum to a production well, little or no petroleum can be recovered from the formation because the viscosity of the petroleum is so great that it will not move. It is generally recognized that if the API gravity of the petroleum contained in the subterranean formation is less than about 12.degree., little or no recovery of the petroleum may be accomplished by conventional primary or secondary recovery means.
It is well known in the art that some improvement in recovery efficiency can be achieved if the temperature of petroleum can be increased. The viscosity temperature relationship of most crude petroleum materials is relatively sharp, and so even though the viscosity may be in the thousands or even millions of centipoise at formation temperatures, by raising the temperature of crude petroleum to several hundred degrees Farenheit, the viscosity may be reduced to a sufficiently low value that it will flow or may be displaced by an extremely dry fluid to a production well where it can be pumped to the surface of the earth.
One of the most extreme examples of viscous petroleum containing formations from which essentially no production may be had by conventional primary or secondary means are the so called tar sand or bitumen sand deposits such as those located in the western United States, and the Athabasca Tar Sands in the northern portion of the province of Alberta, Canada, as well as in Venezuela. The viscosity of the bituminous petroleum contained in the Athabasca tar sand deposits, for example, is in the range of several million centipoise at the average formation temperature of about 40.degree. F., and so the bituminous petroleum is essentially immobile at these temperatures. Additionally, the permeability of the Athabasca tar sand deposit is so low that the injection of a heating fluid thereinto is quite difficult, and so recovery of the bituminous petroleum from these deposits by any means other than mining has been essentially unsuccessful on any commercial level up to the present time.
One means which has been described in the prior art for increasing the mobility of high viscosity, low API gravity crudes is by means of in situ combustion, also referred to on the literature as fire flooding. In situ combustion is accomplished by introducing heated air into the formation, generally the temperature of the air being above about 350.degree. F., as a result of which a combustion reaction is initiated within the formation. Continued injection of air into the formation after the initiation of this combustion reaction result in the propagation of this combustion front through the formation, heating and displacing crude petroleum ahead of it.
In application of a typical forward in situ combustion operation as applied to a subterranean formation which is penetrated by at least two wells, one for the purpose of injecting air or some other oxygen containing gas, and at least one for the purpose of recovering petroleum from the formation, an oxygen containing gas, generally air, is introduced into the formation via the injection well, and the combustion reaction is initiated in the petroleum saturated formation immediately adjacent to the injection well bore by means of any one of several known means, such as by the use of a gas fired downhole heater or a downhole electric heater or by chemical means. All of these injection techniques are well described in the literature. Once the ignition of a portion of the formation has been accomplished, additional heating is frequently not necessary, and the combustion reaction will be sustained by continued injection of the oxygen containing gas to propagate the combustion front through the formation.
As the combustion front advances through the formation, a swept area consisting ideally of clean sand is left behind the front. In advance of the combustion front and moving generally radially outward away from the injection well and predominantly in the direction of the production well, various sequential zones are formed. A distillation and cracking zone is formed immediately ahead of the combustion front, and in front of this zone is a condensation and vaporization zone, and finally an oil bank of heated petroleum is formed. The temperature of the combustion zone is generally in the range of from about 650.degree. to about 1200.degree. F. The heat generated in the zone is transferred to the distillation and cracking zone ahead of the combustion where the crude petroleum undergoes distillation and cracking. The temperature in the distillation and cracking zone is in the range of from about 300.degree. to about 450.degree. F.
Ahead of the distillation and cracking zone is a condensation and vaporization zone, which has a temperature of from about 200.degree. to about 450.degree. F., depending on the distillation characteristics of the petroleum contained therein and the pressure existing within that particular zone. Fluids present in this zone include water and steam as well as hydrocarbon components of the crude.
Ahead of the condensation and vaporization zone is an oil bank which forms as the in situ combustion front progresses through the formation and the formation crude is displaced toward the production well. This zone contains not only heated reservoir crude petroleum, but also condensate, cracked petroleum components, and some gaseous products of combustion which will ultimately pass through the production well along with crude petroleum.
Significant recovery stimulation has been experienced by application of conventional in situ combustion processes to moderately viscous petroleum. These techniques have not been altogether successful when applied to varied viscous petroleum, however, particularly bituminous petroleum such as is contained in the tar sand deposit. The build up of the oil bank ahead of the thermal front is believed to be a source of failure of fire flooding in tar sand deposits, since the unaltered petroleum present in the oil bank will become essentially immobile if it looses appreciable amounts of heat, which will occur as the bank moves into cooler portions of the formation.
Another source of failure of commercial exploitation of viscous petroleum formations by means of conventional in situ combustion is associated with the occurrence of zones or pockets of in situ combustion within the formation for several reasons. The zone of combustion initiated by the pockets of in situ combustion by spontaneous combustion may move in a direction opposite of that in a normal forward in situ combustion process, i.e., the zone may move toward the injection well, and thus consume the oil which has been mobilized by any other combustion fronts which have been established and caused to move in a direction from the injection well toward the production well. Similarly, the zone may remain essentially stationary, and thus consume a substantial portion of mobilized petroleum which moved into it as the desired combustion front progresses toward the production well.
For the above described reasons as well as other reasons which may not be entirely understood at the present time, the recovery of very viscous petroleum from subterranean formations such as tar sand deposits by means of in situ combustion has not been especially satisfactory, from the standpoint that the percentage of the oil removed has been quite low. Accordingly there is substantial need for a means of conducting an in situ combustion reaction so that no random pockets of spontaneous ignition occurs within the formation, and so as to achieve a significant increase in oil recovery from the formation.